#include "common.h"

/**
 * @a https://leetcode.cn/problems/swap-nodes-in-pairs/
 */

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution
{
public:
    ListNode *swapPairs(ListNode *head)
    {
        if (head == nullptr || head->next == nullptr)
            return head;

        ListNode *newhead = new ListNode(-1, head);
        ListNode *prev = newhead;
        ListNode *cur = head;
        ListNode *next = cur->next;
        ListNode *nnext = next->next;
        while (cur && next)
        {
            cur->next = nnext;
            next->next = cur;
            prev->next = next;

            prev = cur; // 顺序别变
            cur = nnext;
            if (cur)
            {
                next = cur->next;
                if (next)
                    nnext = next->next;
            }
        }
        ListNode *ret = newhead->next;
        delete newhead;

        return ret;
    }
};
class Solution
{
public:
    ListNode *swapPairs(ListNode *head)
    {
        if (head == nullptr || head->next == nullptr)
            return head;
        ListNode *newhead = head->next;
        head->next = swapPairs(newhead->next);
        newhead->next = head;
        return newhead;
    }
};